Field of the Invention
The present invention relates to an image forming apparatus using electrophotography.
Description of the Related Art
In an image forming apparatus using electrophotography, the surface of a photosensitive drum is charged by a charging roller, and the charged photosensitive drum is exposed by an exposing apparatus, whereby an electrostatic latent image is formed on the photosensitive drum. The electrostatic latent image is developed as a toner image by a developing apparatus, and the toner image is transferred to a recording medium by a transfer roller. Then the toner image transferred to the recording medium is fixed to the recording medium by a fixing apparatus. In this way, an image is formed on the recording medium.
In the case of the image forming apparatus using electrophotography, a charging roller is often used as a means of stably charging the photosensitive drum. The charging roller rotates while contacting the photosensitive drum. The photosensitive drum is charged by a discharge, which is generated in the gap between the charge roller and the photosensitive drum near the contacting part of the charging roller and the photosensitive drum.
If a voltage, higher than a voltage that causes a discharge between the charging roller and the photosensitive drum (discharge start voltage Vth) is applied to the charging roller, a discharge is generated between the charging roller and the photosensitive drum, and the potential of the surface of the photosensitive drum increases. The potential of the surface of the photosensitive drum increases in proportion to the voltage applied to the charging roller. In concrete terms, for example, to set the potential of the surface of the photosensitive drum to a target value Vd, a voltage of “the target value Vd+the discharge start voltage Vth” must be applied to the charging roller. However, if a film thickness of a photosensitive layer becomes thin due to deterioration of the photosensitive drum, the discharge start voltage Vth drops. In the case when the voltage applied to the charging roller is constant, a drop in the discharge start voltage Vth increases the potential of the surface of the photosensitive drum.
Therefore with a technique disclosed in Japanese Patent No. 5511891, the film thickness value of the photosensitive layer of the photosensitive drum is acquired, and, based on the film thickness value of the photosensitive layer, the potential of a non-image forming portion, where no toner image is formed on the photosensitive drum (hereafter called “non-image portion”), is set to a target value. In concrete terms, the charging roller charges the photosensitive drum so that the potential of the entire surface of the photosensitive drum becomes a target value or higher. Then the absolute value of the potential of the non-image portion is lowered by the exposing apparatus exposing the non-image portion at a first output, so that the potential of the non-image portion becomes a target value. Then an electrostatic latent image is formed on the surface of the photosensitive drum by the exposing apparatus exposing at a second output an image forming portion which is the portion on the photosensitive drum where a toner image is formed (hereafter called an “imaging portion”).
If the exposing apparatus exposes the surface of the photosensitive drum here, plus charges are generated in a charge-generating layer in the photosensitive drum, as shown in FIG. 19. The plus charges migrate to the surface of the photosensitive drum through a charge-transporting layer. If the surface of the photosensitive drum is exposed with this mechanism, the absolute value of the potential of the surface of the photosensitive drum becomes small. Moreover, when the voltage applied to the charging roller is constant, the potential of the photosensitive drum is also changed by the humidity of the location where the image forming apparatus is used. Therefore according to a technique disclosed in Japanese Patent Application Laid-open No. 2000-187363, the potential of the image portion in the photosensitive drum is set to be constant regardless of the humidity of the location where the image forming apparatus is used.
In the case of the technique disclosed in Japanese Patent No. 5511891, due to high humidity, the discharge amount generated between the photosensitive drum and the charging roller becomes high in a high temperature/high humidity (H/H) environment (temperature: 30° C./humidity: 80%), compared with the case when humidity is low. If the charge amount generated between the photosensitive drum and the charging roller increases, a friction force between the photosensitive drum and a cleaning blade increases. This may result in the minute vibration of the cleaning blade, which may generate an abnormal sound.
In a low temperature/low humidity (L/L) environment (temperature: 15° C./humidity: 10%), due to low humidity the discharge amount generated between the photosensitive drum and the charging roller decreases compared with the case when humidity is high. Moreover, when humidity is low, a hardness of the cleaning blade increases, which makes the contacting state of the surface of the photosensitive drum and the cleaning blade unstable. This may result in toner falling through gaps between the surface of the photosensitive drum and the cleaning blade, which may cause the adherence of toner to the charging roller. Toner that adheres to the charging roller is mainly toner which was not transferred from the photosensitive drum to an intermediate transfer belt or the like. This toner is mainly charged to the positive polarity.
If humidity is low here, the discharge amount generated between the surface of the photosensitive drum and the charging roller becomes low, as mentioned above. Therefore the toner remaining on the photosensitive drum is not sufficiently charged to the negative polarity, and adheres to the charging roller to which the negative polarity voltage is applied. If the toner charged to the positive polarity adheres to the charging roller, the potential of the part on the charging roller where the toner adheres becomes unstable. Because of this, in some cases the photosensitive drum cannot be appropriately charged by the charging roller.